Lead azide (“LA”) has been widely used in ordnance systems for many years. Virtually all chemical detonators utilize LA as the initial shock wave generating compound necessary for proper detonation of subsequent main explosive charges. LA is a reliable explosive material and because it has been studied extensively, properties and manufacturing processes are well defined. Despite being a useful energetic material. LA contains lead, a toxic heavy metal that is released to the environment during production and use. LA also has shortcomings in that it is unstable in non-hermetic munitions applications and decomposes to hydrazoic acid gas in the presence of water and CO2. This hydrazoic acid may react with metal components elsewhere in the munition to form unstable metal azides such as copper azide.
Various other inorganic aides have been considered for use as detonants. Of these, only silver azide (SA) has properties that make it suitable for general usage. In addition, silver azide has a number of advantages over LA that make it appealing for use in commercial detonators. Silver generated from use of silver azide is generally considered less toxic from both environmental and human health perspectives relative to lead. Silver azide also generates a very low partial pressure of hydrazoic acid when exposed to water and CO2, precluding formation of other metal azides that may he considered hazardous. Most importantly, silver azide demonstrates superior chemical stability and has enhanced detonation properties compared to lead azide.
A number of methods have been utilized to prepare silver azide, the majority involve reaction of sodium azide with a water soluble silver salt such as silver nitrate. “Energetic Materials”, Edited by H. D. Fair, R. F. Walker, New York, Plenum Press, Vol. 2 (1977), p. 11.
The low solubility of silver azide and propensity for profuse nucleation leads to a colloidal product with low bulk density and poor handling characteristics when silver azide is prepared by this unmodified process. Synthesis of silver azide utilizing sodium hydroxide and, later, ammonium hydroxide, increased the solubility of the silver azide formed and provided a larger, crystalline product via slow addition of acid (RD1336). GB Patent 887,141; GB Patent 781,440; G. W. C. Taylor, “The Manufacture of Silver Azide RD1336” Report 2/R/50 (Accession No. ADA 474242), Explosives Research and Development Establishment, Waltham Abbey, 1950.
Current methods of silver azide production typically use a “Costain” process, which provides a product with reasonably consistent particle size and morphology, good handling characteristics, and a high bulk density. U.S. Pat. No. 3,943,235. The Costain process involves addition of an aqueous sodium azide solution to a solution of silver nitrate and ammonium hydroxide in water. The ammonium ion coordinates with the silver to provide a soluble silver complex. The mixture is heated to 75° C., and the ammonia is slowly distilled off, breaking down the complex and thereby supplying silver at a slow, controlled rate. At the first appearance of precipitation of silver azide, a dilute solution of acetic acid is introduced to neutralize free ammonia and induce formation of seed crystals. The temperature is then ramped to 90-95° C. to remove additional ammonia, and the reaction volume is maintained by addition of water. During the distillation of ammonia, additional silver azide crystals form. The reaction is terminated by cooling, and residual ammonia is neutralized with dilute acetic acid. The product is isolated by filtration to afford a free-flowing granular silver azide product. Typically, particle size and morphology of the product may be controlled to some extent by altering reaction concentration, stirring rate, and/or reactor configuration.
Further modifications of the initial Costain process have been made to control the nucleation process and thus the properties of the silver azide produced. These include use of acids other than acetic acid or addition of modifiers such as sodium carboxymethylcellulose to better control nucleation rate or crystal habit. R. McGuchan, “Improvements in Primary Explosive Compositions and their Manufacture”, Proceedings of the 10th Symposium on Explosives and Pyrotechnics, San Francisco, Calif. (USA), 1979; M. van der Merwe, “The Preparation and Chemical and Physical Characterization of Silver Azide,” Proceedings of the 12th Symposium on Explosives and Pyrotechnics, San Diego, Calif. (USA), 1984.
The Costain and modifications of the Costain method for preparation of silver azide do not appear to be optimal for preparation of a highly consistent silver azide product that exhibits high thermal stability for use in commercial applications. High purity silver azide that is chemically stable to 500° F. may be produced on a small scale utilizing the Costain process; however, scale-up to the ≥100 gram level seems problematic. These issues appear to be related to inconsistent morphology that is often a result of the process including accelerated crystallization at the reaction-air interface and volume and associated temperature) changes during the distillation of ammonia. Plating of silver azide on reactor walls and stirring apparatus also typically occur. The consequence may be a lower purity product with attenuated thermal stability. Addition of a crystal modifier may provide reproducible morphology but may lower the thermal stability as well.
As a result, there is a need to improve the reproducibility of the silver azide manufacturing process. Historically, modifications of the silver azide process have sought to reduce rapid nucleation pH adjustment prior to and/or during crystallization and increasing particle size, bulk density, and providing consistent morphology by decreasing the rate of ammonia evolution. A new method has been developed which controls these properties of the silver azide product not by ammonia evolution (decoction of the silver-ammonium complex) but via hydrolysis of an appropriate ester to form ammonium acetate. This new procedure utilizes ethylene glycol diacetate or other related esters to neutralize the ammonium ion. The reaction is initially biphasic with reactants in an aqueous phase that is in contact with an organic phase containing the ester. The reaction is heated (50° C.-90° C.) with stirring and as the ammonia is neutralized, the reaction becomes monophasic (since the hydrolysis product, ethylene glycol monoacetate is water soluble) and silver azide is formed. Evolution of ammonia gas is minimized during the process to afford more kinetic control of the crystallization and avoid issues related to the Costain process.